This invention relates to an electronic circuit for dynamic control of filtering and phase shift.
In many applications, such as in synthesizing sound or processing audio or analog signals, it is desirable to employ filters having variable phase shift or cut-off frequencies. While some control can be achieved in a filter circuit using variable resistors or capacitors, some noise and distortion may be experienced, or the circuit may be too expensive.
The classic phase-shift circuit is comprised of a differential operational amplifier with the input resistor, R.sub.A, nominally equal to the feedback resistor, R.sub.B. When a signal V .sub.in = V sin .omega.t is applied to the input, the output is: EQU V.sub.out =V sin [.omega.t+.delta.], 1.
where .delta. is equal to arctan [2R.sub.c C.omega./(R.sub.c.sup.2 C.sup.2 .omega..sup.2 -1)] and R.sub.c is an input resistor connecting V.sub.in to the noninverting input of the amplifier, and the capacitor, C, is connected between that input terminal and circuit ground. Such phase shift stages can, of course, be cascaded to give arbitrarily large phase shifts.
Mixing the output of such a network with the original signal in a 1:1 ratio results in a comb filter response due to cancellation at all frequencies whose phase shift corresponds to an odd integral multiple of 180.degree.. Dynamically shifting the location of the comb filter in the frequency spectrum produces a very interesting audio processing effect commonly called phasing. This is the major but not the only application of a dynamically controllable phase-shift circuit.
Traditionally, dynamic control has been effected by controlling the value of coupling resistor R.sub.c by various sorts of voltage controlled resistors. One approach has been the use of light variable resistors. This particular approach has the disadvantages of irreproducability and unreasonable expense. A lower cost approach is to use a field-effect transistor (FET) for R.sub.c. This requires selection of FETs for proper characteristics, particularly when cascaded stages are used, and also gives noticeable distortion. The distortion may be compensated by several means, but these add to the cost of the circuit. In another approach to electronic control of phase shift disclosed in U.S. Pat. No. 3,475,623, the exponential volt-ampere characteristic of the base-emitter junction of a silicon junction transistor is employed as a voltage controlled resistor to provide a variable R.sub.c network, but that will not satisfy the need for an inexpensive, simple circuit capable of being easily implemented with either discrete components or integrated circuits, particularly in the case of an all-pass phase-shift circuit.